Low-gassing high strength battery separator and method of production

ABSTRACT

Production of inorganic porous sintered battery separator substantially reducing formation of gas when in contact with a zinc electrode, and having a dendrite formation inhibiting effect on such zinc electrode, permitting long cycle life of a high energy density battery such as a silver-zinc or nickel-zinc battery, and providing a sealed battery, produced by firing or sintering a compacted magnesium silicate-iron silicate composition, e.g., derived from the mineral olivine, and containing combined lead, e.g., as lead silicate, at temperature ranging from about 1,000*C to about 1,300*C in non-oxidizing atmosphere, e.g., a carbon monoxide-carbon dioxide atmosphere, to produce a porous sintered membrane consisting essentially of magnesium silicate, iron silicate and combine lead, e.g., in the form of lead silicate, the iron content thereof being substantially entirely in ferrous form.

Tlit States Smatlto i QM [19] [54] LOW-GASSING GH STRENGTH BATTERYSEPARATOR AND METHOD OF PRODUCTION Inventor: Joseph S. Smatko, SantaBarbara,

Calif.

Assignee: McDonnell Douglas Corporation,

Santa Monica, Calif.

Filed: Oct. 19, 1970 Appl. No.: 81,827

References Cited UNITED STATES PATENTS 5/1969 Arrance et a] ..l36/6 May22,1973

[57] ABSTRACT Production of inorganic porous sintered battery separatorsubstantially reducing formation of gas when in contact with a zincelectrode, and having a dendrite formation inhibiting effect on suchzinc electrode, permitting long cycle life of a high energy densitybattery such as a silver-zinc or nickel-zinc battery, and providing asealed battery, produced by firing or sintering a compacted magnesiumsilicate-iron silicate composition, e.g., derived from the mineralolivine, and containing combined lead, e.g., as lead silicate, attemperature ranging from about l,000C to about 1,300C in non-oxidizingatmosphere, e.g., a carbon monoxide-carbon dioxide atmosphere, toproduce a porous sintered membrane consisting essentially of magnesiumsilicate, iron silicate and combine lead, e.g., in the form of leadsilicate, the iron content thereof being substantially entirely inferrous form.

22 Claims, 1 Drawing Figure Patented May 22, 1973 W v mw P655 14 6'.SMATKO INVENTOR.

,QTTOQAIE v BYw LOW-GASSING HIGH STRENGTH BATTERY SEPARATOR AND METHODOF PRODUCTION This invention relates to batteries, particularly highenergy density batteries and is especially concerned with the productionof improved inorganic membranes or separators for use in such batteries,especially a high energy density battery containing a zinc electrode,such separators having substantially reduced tendency to cause gassingwhen in contact with a zinc electrode, thereby permitting production ofhermetically sealed batteriesof this type having extended life, whichcan operate as a secondary battery over a large number ofcharge-discharge cycles efficiently; with novel procedure for producingsuch separators; and with improved battery constructions embodying suchimproved separators.

This application is a continuation-in-part of my copending application,Ser. No. 70,400, filed Sept. 8, 1970, entitled LOW-GASSING BATTERYSEPARA- TOR AND METHOD OF PRODUCTION.

Batteries are an important source of energy storage for powergeneration. Important types of battery particularly suited for suchapplications are the high energy density alkaline electrolyte cells suchas the silverzinc, zinc-air and nickel-zinc batteries. High energydensity batteries are generally battery systems which have asubstantially higher energy per unit of weight than conventional, e.g.,lead, storage batteries. In addition to important airborne applications,such high energy density batteries have many other applications such asin portable tools and appliances, television, radio and record players,engine starting, portable X-ray units and the like.

In high energy density batteries such as silver-zinc and nickel-zincbatteries, the electrodes are placed adjacent opposite sides of amembrane or separator which performs the function of retainingelectrolyte, separating the electrodes, and permitting transfer ofelectrolyte ions while inhibiting migration of electrode ions. Foractivation of these batteries, the battery or the components thereofsuch as the separator are filled with an aqueous alkaline electrolyte inthe form of an aqueous solution of an alkali such as potassiumhydroxide.

High energy density batteries of the above type, particularly thoseemploying an inorganic separator, are particularly useful as secondarybatteries which can be charged and discharged periodically, and canoperate at elevated as well as at normal temperatures.

One form of particularly useful inorganic separator for such high energydensity batteries, such as silverzinc batteries is disclosed in U.S.Pat. No. 3,446,668. Such inorganic separator is in the form of asintered porous member composed of magnesium silicate and iron silicate.According to the patent, such separators in addition to being formedfrom synthetic mixtures of ironbearing material, magnesium-bearingmaterial, and silica, can be formed from the naturally occurring mineralolivine, a magnesium-iron silicate.

Although the magnesium silicate-iron silicate separator of the abovepatent has proved successful in high energy density batteries, includingsilver-zinc batteries when such batteries or cells are vented, it hasbeen found from experience that cells incorporating such separators andcontaining a zinc electrode, eventually develop gas. Thus, whennon-vented or sealed cells such as a sealed silver-zinc batterycontaining the above noted magnesium silicate-iron silicate separator iscycled, such batteries eventually develop gas, although such cells canbe cycled from about 12 to about cycles on shallow cycling regimeswithout excessive pressure rise. However, thereafter such cells gassignificantly during overcharge, and on standing, and in due course oftime the pressure rise is sufficiently great to present the danger ofrupture of the battery case. Analysis of the gas generated in suchbatteries shows the majorcomponent to be hydrogen.

Tests have been devised to determine the gassing potential of thevarious cell components. One such test is based on mixing apredetermined weight of test material, such as particulate sinteredmagnesium silicateiron silicate separator material produced according tothe above patent, with a pre-established amount of zinc powder,compressing the mixture and then exposing the compressed pellet to about30% KOH solution. The gas produced is collected and measured over acertain time period, the amount of gas collected providing a measure ofthe degree of activity toward gassing of the above noted separatormaterial in contact with the zinc. This test showed that the magnesiumsilicate-iron silicate separator material of the above patent is veryprone to cause gassing when in contact with zinc.

Attempts were made to de-activate the gassing sites with sulfidetreatment or with quinoline, but these failed to yield a long term andsatisfactory solution to the gassing problem of the magnesiumsilicate-iron silicate separator material. Other attempts involved theapplication of a thin coating of a non-gassing ceramic material such aszirconia on the sintered magnesium silicate-iron silicate, or olivine,separator member of the above patent. At best, this latter techniqueserved only to delay to a minor extent the onset of significant gassingwhen such coated separator was used in a silver-zinc cell, but did notprovide a satisfactory solution to the problem.

It was found that after sintering or firing the raw material olivine ina gas fired kiln to produce the sintered porous separator, according tothe procedure of the above U.S. Pat. No. 3,446,668, the originaloff-white colored olivine changed to a deep red-brown color, indicatingpartial decompositon and the oxidation of a portion of the ferrous ironin the olivine to Fe O and small amounts of Fe O depending upon thefiring conditions. The presence of ferric compounds in the resultingseparator is noted in the patent. It was found that these higheroxidation states of the iron in the olivine, produced upon firing theolivine in a gas-fired kiln in the presence of combustion gases and air,when in contact with zinc promotes dissolution of the zinc withliberation of hydrogen gas at marked rates.

It was discovered according to the invention of my above copendingapplication that if the magnesium silicate-iron silicate composition, orolivine, e.g., in compacted form, is sintered or tired in a controlledatmosphere containing a reducing gas, particularly a combination ofcarbon monoxide and carbon dioxide (hereinafter also referred to asCO/CO gases, or an inert gas, under conditions to maintain the ironcontent of the starting mixture in the ferrous form with substantiallyno ferric oxides produced, the resulting sintered magnesiumsilicate-iron silicate separators have substantially lower gassing rateswhen in contact with a zinc electrode as compared to separators producedfrom the same starting materials but by sintering in the presence ofoxidizing gases including air, according to the procedure of the abovepatent.

It has now been found according to the present invention that if leadcombined in suitable form, e.g., as lead silicate, is incorporatedpreferably in minor amount in the above noted magnesium silicate-ironsilicate composition, e.g., olivine, and the resulting composition incompacted form is fired or sintered in a controlled type atmosphere,e.g., a CO/CO atmosphere, to maintain the iron content of the abovecomposition in ferrous form, the resulting sintered porous membercontaining combined lead and iron silicate in ferrous form, hassubstantially lower gassing tendency when in contact with zinc, and hassubstantially higher strength, than the porous sintered membrane orseparator produced according to my above copending application. Thus,for example, as will be pointed out hereinafter, gassing rates of theporous membranes or separators produced according to the presentinvention, and containing combined lead, can be less than one-half thegassing rates for the magnesium silicate-iron silicate separators of myabove copending application, and also with a transverse strength ormodulus of rupture which can be double that for the magnesiumsilicate-iron silicate separators disclosed in U.S. Pat. No. 3,446,668.

Briefly then, the process of the present invention for producing alow-gassing separator when employed with a zinc electrode, and havinghigh strength and good resistance to alkali, is produced by a processwhich comprises sintering a compacted magnesium silicate-iron silicatecomposition containing combined lead, at temperature ranging from about1,000C. to about 1,300C. in a non-oxidizing atmosphere particularly anatmosphere selected from the group consisting of a reducing gas and aninert gas atmosphere, under conditions to maintain the iron content ofsuch composition in the ferrous form.

A particularly effective reducing gas atmosphere for carrying out theabove noted sintering to maintain the iron content of the composition inferrous form, is a properly balanced mixture of carbon monoxide andcarbon dioxide, hereinafter also denoted a CO/CO atmosphere, and aneffective inert gas atmosphere being argon.

In carrying out the present invention for producing the lead-containingmagnesium silicate-iron silicate porous separators of the invention, acompacted starting mixture of olivine or equivalent synthetic mixturedescribed more fully hereinafter, and containing a suitable lead-bearingmaterial or lead compound as described more fully below, is firstsubjected to firing or sintering preferably in a controlled or balancedreducing or inert atmosphere described above, to cause reaction betweenthe components of such mixture to form a magnesium silicate-ironsilicate containing lead combined therewith, i.e., in the form of leadsilicate.

The resulting reacted mixture in compacted form is then again sinteredin a reducing or an inert atmosphere, e.g., a CO/CO or argon atmosphere,to maintain the iron content of the composition in ferrous form, usuallyat a lower sintering temperature than the temperature of the initialfiring operation, to develop the desired structure of the separator interms of high strength, desired porosity and low resistivity, and havingthe resulting important property of being very lowgassing when incontact with a zinc electrode in a battery, to provide a battery havinga long cycle life yet which can be hermetically sealed.

in carrying out the invention for obtaining the improved lead-containingmagnesium silicate-iron silicate separator hereof, a synthetic mixtureof suitable ironbearing, magnesium-bearing, lead-bearing and SiO,-bearing material is formed, generally in proportions by weight of about1 percent to about 65 percent preferably about 1 percent to about 26percent, of iron or ironbearing material or compound calculated asferrous oxide (FeO); about 4 percent to about 56 percent,

preferably about 14 percent to about 56 percent, of magnesium ormagnesium-bearing material or compound calculated as MgO, about 0.4percent to about 69 percent, preferably about 1 percent to about 48percent, lead or lead-bearing material or compound calculated as PbO,and about 15 percent to about 43 percent, preferably about 24 percent toabout 32 percent, of silica (SiO Suitable iron-bearing materials or ironcompounds which can be employed include, for example, ferrous or ferricsalts, such as the sulfate or chloride, ferrous or ferric oxide, ormetallic iron powder, or mixtures of the above ferrous or ferriccompounds, and iron powder. Also, there can be employed naturallyoccurring iron minerals such as magnetite, hematite, Siderite, Fayelite,rouge, and particularly olivine.

As sources for suitable magnesium-bearing materials or magnesiumcompounds, there can be employed magnesium carbonate, magnesium sulfate,magnesium nitrate, magnesium chloride, magnesium oxide, and thenaturally occurring minerals talc, Enstatite, Magnesite, Forsterite, andparticularly olivine.

As noted above, olivine can be employed as source of both the iron andmagnesium of the starting mixture.

As source for the lead, or lead-bearing material or compound, there canbe employed lead acetate, lead dioxide, white lead, red lead, litharge,lead hydroxide, tribasic lead silicate, lead sulfate, lead powder, basiclead acetate (also known as lead subacetate) and lead nitrate. Even leadsulfide (galena) may be used during the initial or oxiding firing.Natural minerals such as Cerrusite (PbCO Anglesite (PbSO,), and thelike, also can be used, if desired.

As a source of suitable SiO -bearing material, there can be employed forexample, flint, silica, sand, diatomaceous earth or magnesium silicate,silica gel, silicic acid, fume silica, and the like.

The starting mixture of iron-bearing, magnesiumbearing and lead-bearingmaterials and silica, can be produced, for example, by first forming asolution of the lead compound, e.g., lead acetate or lead nitrate in asuitable solvent, such as water or an organic solvent, e.g., methanol,ethanol, and the like, where such lead compound is water or solventsoluble, and the solution of lead compound then added to the othercomponents, e.g., olivine or a synthetic mixture of the magnesium andiron-bearing materials, and silica-bearing material, to yield a thickmixture or paste. The mixture is stirred to evaporate at least a portionof the solvent, to form a crumbly damp mass. Where the lead-bearingmaterial or compound is insoluble, the latter material can be mixed withthe other above noted components, e.g., by ball milling in the presenceof water, and the mixture filtered and dried, and the dried mixture isthen crumbled. Preferably, there is added to the crumbly mass formed byeither of the above procedures, a small affin wax, generally dissolvedin a suitable solvent such as acetone or toluene, and the resultingmixture dried, e.g., at ambient temperature or at elevated temperatureto remove solvent. Although the addition of an organic binder is notnecessary, it is preferred to employ such binder to provide adequategreen strength for handling.

The resulting crumbly mass is then pressed into blocks at pressures ofabout 2,000 to about 20,000 psi, the presence of the binder aiding inthis operation. Such blocks are then fired first in air at temperatureof the order of about 400C. to about 600C. for a period of time of about15 minutes up to about 4 hours, removing the organic binder, and thenfired at temperature of the order of about l,l00C to about 1,400C. Thelatter firing operation preferably is carried out in the presence of areducing atmosphere, e.g., a CQ/CO, atmosphere or an inert argonatmosphere, to maintain the iron content of the mixture in the ferrousform and to convert any ferric species to the ferrous form. However,this initial firing operation alternatively can be carried out in air,relying on the subsequent sintering operation pointed out more fullybelow, to maintain the iron content of the mixture in the ferrous formand to convert any ferric species to the ferrous form.

The initial sintering operation noted above generally is carried out fora period of about 1 to about 8 hours.

Such initial firing operation causes reaction between the variouscomponents of the mixture to convert the mixture into a magnesiumsilicate-iron silicate, containing the lead component in combined form.The lead component according to present information is also believed tobe in the form of a lead silicate. During such initial firing operation,at the high temperatures, the various components, e.g., the magnesium,iron and lead-bearing materials or compounds, are converted to theoxides and then by reaction forming with the silica component, themagnesium-iron-lead silicate having the general formula:

The values for a, b and c subscripts of the above formula vary dependingupon the relative proportions of the magnesium-ironand lead-bearingmaterials, and silica employed.

The resulting initially fired pressed blocks, following cooling thereof,are granulated or pulverized, and the granular material can beball-milled with water or other liquid medium such as acetone, thematerial filtered and the wet cake mixed with a solvent solution, e.g.,an acetone solution, of an organic binder of the types noted above, suchas Carbowax, in an amount noted above. The resulting material is thendried and the resulting crumbs are granulated and the particles furtherreduced in size, e.g., by pressing granulation, as through a screen. Theresulting powder is then pressed into blocks or plaques at pressures,e.g., ranging from about 2,000 to about 20,000 psi, and fired orsintered, e.g., in an electric furnace, first in air at temperature ofthe order of about 400C to about 600C for a period of about 15 minutesto about 4 hours, to remove the organic binder, followed by sintering attemperature of the order of about 1,l00C to about 1,300C for a period ofabout 10 minutes to about 6 hours in a reducing atmosphere such as aCO/CO or in an inert, e.g., argon atmosphere, to maintain the ironcontent of the compressed plaque in the ferrous form or to convert anyferric species thereof to the ferrous form.

Following the firing or sintering operation in a controlled, e.g., CO/C0or argon atmosphere, the separators are maintained in this reducingatmosphere in the furnace during the cooling period down to about 500Cto about 300C. The reason for this is that at these high temperatures ofabout 1,300C down to about 300C, at least some of the ferrous content ofthe fired separators would be converted to ferric oxides in the absenceof such reducing atmosphere, which is specifically avoided in order toobtain the improved low-gassing separators, and hence such coolingbetween about l,300C down to about 500 to 300C is carried out in thesame controlled, e.g., CO/CO, or argon atmosphere, present during suchfinal firing or sintering operation. Thereafter, the separators arecooled rapidly. When employing a CO/CO atmosphere, upon reaching about500C one can turn off the CO/CO and substitute an inert gas such asargon or nitrogen down to a temperature of 300C and then air can beadmitted. The resulting sintered separators are generally in the form ofsolid solutions of magnesium silicate, ferrous silicate and the combinedlead, believed to be lead silicate. The resulting separators formedfollowing the second sintering operation in either a reducing or aninert gas atmosphere, have a composition ranging from about I to about99 mol percent magnesium silicate, about 1 to about mol percent ferroussilicate, and

about 0.1 to about 50 mol percent combined lead as lead silicate, suchranges preferably being from about 45 to about 98 mol percent magnesiumsilicate, about 2 to about 30 mol percent ferrous silicate, and about0.2 to about 25 mol percent of combined lead as lead silicate.

The second sintering operation carried out, e.g., in a CO/CO atmosphereor an argon atmosphere, in addition to maintaining the iron content ofhe initially fired magnesium-iron-lead silicate in the ferrous form withsubstantially none of such iron content being converted to the highervalence ferric species, thus resulting in a very low gassing separatorin the presence of a zinc electrode, also develops the structure of theseparator, and it has been found unexpectedly that the resultingseparator containing lead has an unusually high transverse strength ormodulus of rupture ranging from about 5,000 up to about 20,000 psi,often in the very high strength range of between about 15,000 and about20,000 psi, which is an unusually high transverse strength for aninorganic battery separator. in addition, the types of initial startingcomponents and the sintering temperature, particularly the temperatureof the second sintering operation, can be varied to provide a porositycorresponding to a water absorption ranging from about 5 percent toabout 50 percent. In addition, it has been found that the presence oflead as a component of the starting mixture in the final separator,according to the invention, inhibits formation of zinc dendrites when incontact with a zinc electrode in a battery, as well as verysubstantially reducing gassing when in contact with a zinc electrode,which not only permits production of hermetically sealed batteries, butpermits higher charging efficiency of the zinc electrode and greatlyenhancing the cycle life of high energy density batteries containing azinc electrode such as the silverzinc and nickel-zinc batteries. Thehigher electrical efficiency of the lead-containing magnesiumsilicateferrous silicate separators of the invention further resultsfrom the low resistivity of the separators produced according to theinvention, which can range from about to about 50 ohm-cm and suchseparators also have high resistance to alkali.

The magnesium silicate-iron silicate-lead silicate separator materialproduced according to the invention, and converted or granulated to aground particulate form, can be employed as inorganic separator materialused in flexible separators. These include, for example, the flexibleseparators described in the copending application, Ser. No. 676,224,filed Oct. 18, 1967 ofF. C. Arrance, et al., now abandoned andconsisting, for example, of a porous inorganic material, which can bethe above noted magnesium silicate-ferrous silicate-lead silicateimproved separator material of the present invention, and a minorportion of a water coaguable organic fluorocarbon polymer such as avinylidene fluoride polymer, to bond the particles of the inorganicmaterial.

Also, the above noted particulate magnesium silicate-iron silicate-leadsilicate separator material produced according to the invention can beemployed as the inorganic material in the flexible separators describedin the copending application, Ser. No. 676,223, filed Oct. 18, l967, nowU.S. Pat. No. 3,542,596 of F. C. Arrance, and consisting for example ofa major portion of such inorganic material, e.g., the above notedmagnesium-ferrous-lead silicate separator material of the invention, aminor portion of potassium titanate, and a minor portion of a curedorganic polymer such as polyphenylene oxide as bonding agent.

Further, the improved inorganic separator material of the presentinvention can be applied in the copending application, Ser. No. 707,808,filed Feb. 23, 1968, of F. C. Arrance, et al., now abandoned in favor ofcontinuation application Ser. No. 154,218, filed June 17, 1971,disclosing an improved flexible porous separator, which can bebox-shaped to provide a compartment for a battery electrode, produced byapplying on a flexible porous substrate, such as flexible sheets or matsof vari ous materials including potassium titanate paper, asbestos,aluminosilicate sheets, and the like, a film comprising a mixture of amagnesium silicate-ferrous silicate-lead silicate separator materialaccording to the invention, and an organic polymeric bonding agent ofvarious types, such as polyphenylene oxide, or a fluorocarbon polymersuch as vinylidene fluoride polymer, bonding the particles of theinorganic material together with the bonding agent, and forming a poroussubstantially inorganic separator film on the flexible substrate.

Also, the particulate magnesium silicate-iron silicatelead silicateimproved separator material of the invention can be employed as theinorganic material in producing the flexible microporous separator filmdescribed in copending application, Ser. No. 27,577, filed Apr. 13,1970, by M. P. Strier and J. S. Smatko, which consists essentially of anorganic polymer such as polytetrafluoroethylene, having particles ofsuch inorganic material uniformly distributed in said film.

Although a CO/CO atmosphere for the firing operation according to theinvention has been found particularly effective as a controlledatmosphere, since the proportion of carbon monoxide to carbon dixoide insuch mixture can be readily balanced so as to prevent formation of Fe Oor Fe O by oxidation, or the formation of metallic Fe by reduction asresult of the pres ence of too great a proportion of CO reducing gas,any other reducing gas atmosphere which can be so controlled or balancedto maintain the iron content of the magnesium-iron-lead silicateseparator material in the ferrous form, without formation of ferriccompounds or elemental iron can be employed. Thus, for example, therecan be employed as reducing atmosphere for the firing operation amixture of hydrogen and water vapor, the proportions of hydrogen andwater vapor being controlled, as in the case of the proportions ofcarbon monoxide and carbon dioxide in the above noted CO/CO preferredatmosphere, to prevent oxidation of the ferrous content of the startingmixture to ferric compounds, or the reduction thereof to elemental iron.

Also, when an inert gas atmosphere is employed for firing the magnesiumsilicate-iron silicate-lead silicate composition, such firing can takeplace, for example, in nitrogen, helium, and other inert gases, as wellas in argon.

The following are examples illustrating practice of the invention:

EXAMPLE 1 Balsam Gap natural olivine (A) having the composition 47-49%MgO, 7-9% FeO and 30-4l% SiO and prefired Balsam Gap olivine (B) whichis prefired in a gas furnace with the products of combustion in contactwith it and produced substantially according to the procedure describedin the above U.S. Pat. No. 3,446,668, by firing the Balsam Gap olivineat a temperature of about 1,300C, in the furnace combustion gases, areprovided. A mixture of Balsam Gap natural olivine and 5 mol percent asmuch lead as there is iron in the olivine, in the form of lead oxide, isalso provided, such mixture designated (C). All three of these startingmixtures (A), (B) and (C) are compacted at about 8,000 psi into flatmembers of about 0.033 inch thick, and the respective members formed ofthe two compositions (A) and (B) are each subjected to tiring in (l) amixture ofa l to 1 volume ratio of CO/CO at l,225C for 1.5 hours, (2)argon at l,225C for 2 hours, and (3) air at 1,l00C for 6 hours. Alsocomposition (C) was subjected to firing in a l to 1 volume ratio ofCO/CO at about l,225C for about 1 hour. In those tests employing a CO/COmixture, following the firing operation, the last mentioned mixture isscavenged from the furnace chamber with pure argon at about 550C, andthe furnace cooled rapidly to about 250C, in order to avoid formation ofany significant amounts of iron compounds or elemental iron, other thanferrous iron. Corresponding samples of the two starting materials (A)and (B) are not fired at all.

Following the above noted firing operations in the above variousatmospheres, the various samples subjected to such firing are granulatedand the materials in particulate form are respectively mixed with 68.4percent zinc powder by weight of the mixture, and the respectivemixtures compressed at about 18,000 psi into pellets, and such pelletscontacted under the same conditions with 30% KOH solution at ambienttemperature. Also, the unheated samples of the two starting materials(A) and (B) are similarly formed into pellets and also contacted withthe same concentration KOH solution. Further, a zinc blank is pelletedin the same manner and contacted with the same concentration KOHsolution. The gas produced in each test is collected and measured withrespect to time. i

The materials tested, firing atmosphere, color of the materials bothbefore and after heating, and amount of the gas collected at variousspecified times from the re- EXAMPLE 2 The following are the materialsof a starting mixture and the respective amounts and percentages thereofemployed:

Components Wt. Wt. Grams action of the respective pellets contacted withthe KOl-l fizlfjxgg g 3 solution, are set forth in the table below:Total 100.0 102.5

- TAELEW Color Gas collected (00.) Starting material Firing atmos.Before heating Alter heating 1 hr. 2 hrs. 6 hrs. 24 hrs. 48 hrs Balsamgap olivine (A) Unheated. Off-whit 2.0 3.3 11.1 45 83 Do (JO/(301.. .do4. 4 6.1; 14.9 41 66 o Argon d0.. 8.9 13.1 26.4 62 ill Balsam gapolivine (B) preflred. Not reheatcd Red-brown 66 108 220 431 559 DoCO/CO; (lo 1. 1 2. 4 8.1 58 D0 rgon "do Var deep slate gray 5. 9 13. 741 117 171 Balsam gap olivine (A) 0fl-white Re -brown 114 154 252 467Balsam gap olivine (B) prefire Red-brown" d0 87 139 260 522 Balsam gapolivine containing lead (C 0.5 1. 0 3. 4 14. 1 25. 4 Zinc blank 0.0 0.00.3 1. 5 8. 0

Although the results from the above Table show markedly reduced gassingrates for both the Balsam Gap olivine material (A) and the Balsam Gapolivine prefired material (B) produced according to the above patent, inthe presence of zinc in alkali when such materials are fired in theCO/CO or argon atmosphere, according to my above copending application,as compared to the Balsam Gap olivine prefired material (B) producedaccording to the patent but which is not reheated or retired in suchreducing or inert atmosphere, in the presence of the zinc in alkali, itis noted that the gassing rates for the Balsam Gap olivine containinglead, according to the present invention, fired in CO/CO- atmosphere,had substantially lower gassing rates, that is less than one-half asmuch, as compared to the Balsam Gap olivine material (A) or (B) tired ineither the CO/CO or argon atmosphere, according to my above copendingapplication.

Thus, noting particularly the gassing rates at the 6 hour, 24 hour and48 hour intervals from the table, it is seen that the gassing rates forthe Balsam Gap olivine containing lead fired in the CO/CO atmosphereaccording to the present invention, are 3.4, 14.1 and 25.4,respectively, as compared to the lowest corresponding values of 8.1, 30and 53, respectively, for the Balsam Gap olivine material (A) or (B),fired in either the CO/CO or argon atmosphere.

Note also the remarkable reduction of gassing in the presence of zincfor the lead-containing Balsam Gap olivine material (C) fired in theCOICO atmosphere as compared to the gassing produced when both of thematerials (A) and (B) are fired in air according to the above patent.

It will be noted further that the initial Balsam Gap olivine material(A) and which is unheated, shows relatively low gassing because the ironcontent of such initial material is essentially in the ferrous form.

The minimal gassing rates for the zinc blank shown in the table aboveclearly indicate that the substantial gassing taking place where olivineor a magnesium silicare-iron silicate composition is in contact with thezinc, results from the reaction of the zinc with the ferric iron contentof the olivine or of the magnesium silicateiron silicate composition,and which gassing is substantially reduced according to the inventionand shown in the table above, by firing an olivine material containinglead (C) in a reducing atmosphere such as a properly balanced CO/COmixture.

The lead acetate is dissolved in a liquid containing 15 cc methanol and4 cc water. Then 40 cc more of ethanol is added. This solution is mixedwith the olivine powder to yield a lumpy paste. Separately, 2 cc ofconcentrated ammonium hydroxide is dissolved in 10 cc methanol, and thisis added to the above paste with thorough stirring. After one-half hourof stirring with coincident partial evaporation of the alcohol, there isadded to the crumbly damp mass 10 cc of acetone containing 5.1 gramsCarbowax-4000, understood to be a polyethylene glycol having a molecularweight of about 4,000. This mixture is dried on a hot plate, withcontinuous stirring. The crumbs are pressed into blocks at 18,000 psi.The blocks are fired in air at 500C. for one-half hour, then they arefired in argon for 2 hours at 1,250C.

The cooled blocks are pulverized in a Bico Pulverizer to 16 mesh, andthen the granular material is ballmilled in acetone for 48 hours. Thematerial is then filtered, and the wet cake mixed with 30 cc hot acetonecontaining 5 grams Carbowax-4000. The material is dried, and theresulting crumbs are granulated on screens to yield a pressinggranulation between 60 mesh and 150 mesh. The pressing powder is pressedinto plaques at 8,000 psi, and fired in an electric furnace, first inair at 450C. for one-half hour, and then in argon at 1,150C. for 15minutes, to yield a separator having a porosity corresponding to a waterabsorption of 23.7 percent, and an apparent density of 1.85g/cc. Modulusof rupture is about 6,100 psi. The resulting separator contains aboutmol percent magnesium silicate, about 9 mol percent ferrous silicate andabout 1 mol percent lead silicate.

EXAMPLE 3 The following mixture of components is prepared:

Components Wt. Wt. Grams Balsam Gap Olivine 91.6 250 Litharge (leadoxide) 8.4 22.8

Total l00.0 272.8

The components are milled in a ball mill for 24 hours in water, followedby filtration, and drying. The dry cake is crumbled and sprayed withjust enough water to increase the weight by 5 percent. Then the mass isstored in a jar or sealed plastic bag for 8 hours to uniformlydistribute the water. The crumbs are then pressed into blocks and driedfor 4 hours at C. in air. The blocks are transferred to crucibles havinga fairly good-fitting cover. This crucible is placed inside anotherlarger crucible having pellets of a previously fired batch of comparablecomposition in the annular space. The outer crucible also has asnug-fitting cover. The crucible pack is placed in a bell jar andevacuated to one micron pressure, and then back filled with argon.

The crucible pack is then transferred into an electric furnace whosechamber can be kept filled with argon. Firing is conducted for 1 hour at1,270C. The cooled blocks are crushed in a Bico pulverizer to 16 meshand then reduced to fine size in a ball mill for 48 hours in water. Theproduct is filtered, and the water in the damp cake is displaced withacetone.

The resulting cake is then made into a paste with 13.5 grams ofCarbowax-4000 dissolved in hot acetone. The paste is dried in an ambientair draft in a hood, and the resulting crumbs are granulated on screensto retrieve a pressing granulation between 60 mesh and 150 mesh.Separator plaques are pressed from this powder. One set of plaques isfired at 450C. in air for 15 minutes to remove the volatiles, followedby firing thereafter in argon at 1,200C. for 25 minutes, yieldingseparators of a porosity averaging 14.5 percent water absorption,density of 2.19 g/cc., resistivity in 30% KOl-l of 12.2 ohm-cm. and amodulus of rupture of 13,800 psi.

Another set of plaques is fired as above to eliminate the volatiles, andthen fired in a 1:1 mixture of CO and CO at 1,200C. for 25 minutes,yielding separators averaging 13.7 percent water absorption, density of2.28 g./cc., and modulus of rupture of 14,800 psi. The color of theseseparators was a slightly lighter shade of gray, than those fired inargon. The resulting separators contain about 87.4 mol percent magnesiumsilicate, about 9.69 mol percent ferrous silicate and about 2.91 molpercent lead silicate.

EXAMPLE 4 The following mixture of components is prepared:

Components Wt. Wt. Grams Siderite 12.7 58.0 Cab-O-Sil 26.3 120.2 MgO27.8 126.8 Lead Acetate 33.2 151.7 Total 100.0 456.7

The mineral Siderite (FeCO Cab-OSil, which is a colloidal silica, andthe MgO are ball-milled in hexane for 16 hours to provide an intimatemixture and fine particle size, followed by filtration and drying. Thecake is powdered and made into a medium paste with the lead acetatedissolved in a small amount of hot water. The paste is very thoroughlymixed while heating to assure a very uniform distribution of the leadsalt. Then while mixing, 50 grams of ammonium carbonate dissolved in 100cc of warm water is slowly added to precipitate a basic lead carbonate.Mixing is continued until the mass is dry.

The crumbs resulting therefrom are compressed into blocks aboutone-quarter inch thick at about 10,000 psi are are then fired in a Catmosphere at 500C. to eliminate the volatiles. After cooling, theblocks are packed into the crucible arrangement described in Example 3and fired at 1,240C. for 2 hours in an argon atmosphere. The cooledblocks having a dense porcelain-like character, are pulverized to 16mesh in a Bico pulverizer, followed by ball-milling in acetone for 50hours. The product is filtered, and the acetone in the damp cake isdisplaced by toluene.

This damp cake is then mixed into a paste with grams of binder (:5 partnapthalene A: part beeswax A: part paraffin wax) in cc of hot toluene,the binder weight being approximately 4 percent of the mineral solidsweight. The paste is dried, and the resulting crumbs are granulatedthrough screens to yield a pressing granulation between 60 mesh andmesh. Separator plaques are pressed from this powder at about 8,000 psi.One group of plaques is fired first in air at 450C. for 15 minutes toeliminate the binder, and then fired in argon for 15 minutes at 1,100C.The separators resulting therefrom have a water absorption of 10.8percent average, an average density of 2.74 g/cc. and a modulus ofrupture of 18,700 psi.

Another group of plaques is fired as above to eliminate the volatilesand then fired in a 1:1 mixture of CO and CO at 1,050C. for 20 minutes.The separators have an average water absorption of 11.1 percent, anaverage density of 2.70, and modulus of rupture of 18,100 psi. Suchseparators contain about 80 mol percent magnesium silicate, about 10 molpercent ferrous silicate and about 10 mol percent lead silicate.

The above separators are especially useful for silverzinc cellsundergoing short cycles of deep discharge with significant overcharge(in excess of 5 percent), and for nickel-zinc cells undergoing any cycleregime, especially since the latter must generally be given significantovercharge (sometimes 50-100 percent to be sure of full capacity in thenickel electrode. The impor tant lead content of these separatorsreduces drastically the treeing or dendrite formation of the zincelectrode during rapid recharge and overcharge as well as suppresses thegassing tendency of the zinc electrode.

EXAMPLE 5 147 grams of prefired Balsam Gap olivine powder (material (B)of Example 1) is mixed with 14 grams of lead nitrate dissolved insufficient water to make a thick paste. The paste is mixed while dryingto assure uniform distribution of the salt. The dried crumbs arecompressed into A inch thick blocks at 10,000 psi, and then are brokeninto 1 inch pieces, p aced into the crucible combination disclosed inExample 3 and fired in a 1:1 mixture of CO and CO atmosphere at 1,200C,for 4 hours.

The resulting granules are pulverized in a Bico pulverizer to 16 mesh.The granules, now a very light gray color as compared to the dark redrusty color of the initial (olivine) material (B) are ball-milled for 48hours in hexane, followed by filtration. The damp cake is made into athick paste with 7.5 grams of Carbowax- -4000 dissolved in 50 cc hottoluene. After drying, the crumbs resulting therefrom are granulatedthrough screens to give a pressing granulation between 60 mesh and 150mesh. The pressing powder is pressed into plaques at 7,500 psi, andfired in air at 450C. for 15 minutes, followed by firing in a 1:1 COzCOatmosphere at 1,200C. for 33 minutes to give separators having aporosity averaging 8.1 percent water absorption, and average density of2.65 g/cc. The modulus of rupture averaged 18,000 psi. Another groupfired for 21 minutes at the same temperature and gas atmosphere averaged10.3 percent water absorption, density of 2.50 g/cc. Modulus of ruptureaveraged 15,200 psi.

The latter separators contain about 86.5 mol percent magnesium silicate,about 9.65 mol percent ferrous silicate and about 3.85 mol percent leadsilicate.

Referring to the accompanying illustrative drawing, the latter group ofseparators are assembled in three separate silver-zinc batteries of thetype indicated at 10, each consisting of two silver electrodes 12, andone zinc electrode 14, with the zinc electrode sandwiched between two ofthe above separators 16, one opposite each face of the zinc electrode,such separators contacting the adjacent silver electrodes. The twosilver electrodes are connected via leads 18 to a battery terminal 20,and the zinc electrode is connected via a lead 22 to the batteryterminal 24. The battery is filled with 30% KOl-l.

Nominal capacity of the cell is 1.1 Amp. Hr. Discharge is to 50 percentdepth over 1 hour, and recharge is made in a three hour period with themaximum voltage allowed to rise to 2.09v. In general about 3-5 percentovercharge is supplied to the cell. Cell life times are as follows: 698cycles, 783 cycles and 779 cycles. None of the cells show zinc dendritepenetrations, and all the cells are sealed.

Total All components are ball-milled for 24 hours in water, filtered,dried, remoistened by spraying with water until 6 percent increase inweight occurs, stored over night in a sealed jar, then are pressed intoblocks at 15,000 psi. The blocks are fired in closed crucibles incontact with air and the products of combustion, in a gas fired furnaceat 1,320C. for 6 hours. The cooled blocks are pulverized in a BicoPulverizer to 16 mesh, followed by ball-milling in water for 60 hours.The product is filtered, and thefilter cake is dried.

The cake is made into a thick paste with 16 grams of paraffin wax in hottoluene, followed by drying and granulation through sieves to yield apressing granulation between 60 mesh and l50 mesh. Plaques are pressedat 11,000 psi and fired in air at 450C. for 15 minutes to drive off thebinder, and then fired at l,2lC. for 17 minutes to yield separatorsaveraging 12.7 percent water absorption, density of 2.34 g./cc, andmodulus of rupture of 11,700 psi. Such separators contain about 75 molpercent magnesium silicate, about l9 mol percent ferrous silicate andabout 6 mol percent lead silicate.

EXAMPLE 7 Another batch of material as in Example 2 is prepared exceptthat with the 100 grams of Balsam Gap olivine, 3.4 grams of basic leadacetate is used, dissolved in enough water to make a thick paste, whichis evaporated to dryness while stirring. The dried crumbs are pressedinto blocks and fired in the double crucible arrangement described inExample 3, in an argon atmosphere. The cooled blocks are pulverized to16 mesh, followed by ball milling in water for 50 hours. The product isfiltered and dried. The dried material is made into a thick paste with4.5 grams of Carbowax- -4000 dissolved in hot acetone, dried, and thecrumbs are granulated to a pressing granulation between 60 mesh and 150mesh. Plaques are pressed at 8,000 psi and are fired at 400C. for 20minutes to remove volatiles, then fired at l,205C. in 1:1 COzCOatmosphere for 2] minutes to yield separators with an average waterabsorption of 12.9 percent, and density of 2.33 g/cc, and modulus ofrupture of 11,500 psi.

l4 EXAMPLE 8 The magnesium silicate-ferrous silicate-lead silicateseparator material produced in Example 2 is ground and ball-milled tofine particle size.

This material is then processed according to Example 1 of abovecopending application, Ser. No. 27,577, to produce a flexiblemicroporous separator in the following manner:

An amount of 335 grams of such material ground and ball-milled to fineparticle size is suspended in 225 grams water to form a 60 percentsuspension by weight. An amount of 237.4 grams of Dupont T-30B TFEaqueous emulsion of polytetrafluoroethylene (60.4 percent solidscontent) is added slowly to such suspension.

After about 20 to 30 minutes of stirring, the resulting homogeneousslurry or aqueous dispersion of the magnesium, ferrous, lead silicateand polytetrafluoroethylene is poured on a pyrex glass plate, and isdrawn down by means of a doctor blade set at 0.038 cm (15 mils). Theresulting film is dried initially for about 15 minutes in the draft of alaboratory hood at about 70F, and further dried at ambient roomtemperature for 15 hours. The dried film is then sintered at 360C for 20minutes. About 12 grams of glycerine is added to the slurry prior tocasting, per 100 ml of such mixture, to improve film properties andprevent cracking during the following sintering operation.

The resulting sintered film formed of about 70 percent of the magnesium,ferrous, lead silicate and about 30 percent polytetrafluoroethylene, ishighly flexible, has uniform distribution of the inorganic silicateparticles, and has good stability in aqueous KOH solution at 50 to 100C.

Although the improved magnesium silicate-iron silicate-lead silicateseparators produced according to the invention are particularlyadvantageous when employed with a zinc electrode to reduce gassingtendency as compared to the magnesium silicate-iron silicate separatorsof the above patent, it will be noted that the improved separators ofthe present invention can also be utilized in high energy densitybatteries containing electrodes other than zinc electrodes, for examplein a nickel-cadmium or silvercadmium battery, with advantageous resultsin providing reduction of dendrite growth on prolonged cycling, owing tothe presence of the lead.

In view of the foregoing, it is seen that the invention providesprocedure for producing low-gassing efficient separators, particularlywhen employed with the zinc electrode, permitting the provision ofhermetically sealed long lived secondary batteries, such as efficientsealed silver-zinc and nickel-zinc batteries. In addition, the magnesiumsilicate-ferrous silicate separators of the invention containingcombined lead as lead silicate preferably in relatively small amount,have markedly increased strength, low resistivity, high alkaliresistance and an inhibiting efiect on zinc dendrite formation.

While I have described particular embodiments of the invention forpurposes of illustration, it will be understood that various changes andmodifications can be made therein within the spirit of the invention,and the invention accordingly is not to be taken as limited except bythe scope of the appended claims.

1 claim:

l. A low-gassing battery separator when employed with a zinc electrode,and having high strength and good resistance to alkali, in the form of aporous member having a composition consisting essentially of sinteredparticles of a solid solution of about 1 to about 99 mol percentmagnesium silicate, about 1 to about 90 mol percent ferrous silicate,and about 0.1 to about 50 mol percent lead silicate.

2. A battery separator as defined in claim 1, said separator having aporosity corresponding to a water absorption ranging from about percentto about 50 percent.

3. A battery separator as defined in claim 2, wherein said compositionconsists essentially of about 45 to about 98 mol percent magnesiumsilicate, about 2 to about 30 mol percent ferrous silicate, and about0.2 to about 25 mol percent lead silicate.

4. A battery separator as defined in claim 3, said separator having amodulus of rupture ranging from about 5 ,000 to about 20,000 psi and aresistivity ranging from about 5 to about 50 ohm-cm.

5. A battery separator as defined in claim 1, wherein said compositionconsists essentially of sintered particles of a solid solution ofolivine and lead silicate, the iron content of said olivine beingentirely in ferrous form.

6. A flexible battery separator as defined in claim 1, said sinteredcomposition in particulate form being distributed uniformly in apolymeric organic binder insoluble in aqueous alkaline solutions.

7. in a battery containing a zinc electrode, a lowgassing batteryseparator in the form of a porous member consisting essentially ofsintered particles of a solid solution of about 1 to about 99 molpercent magnesium silicate, about 1 to about 90 mol percent ferroussilicate, and about 0.1 to about 50 mol percent lead silicate.

8. In a battery as defined in claim 7, said composition of saidseparator consists essentially of sintered particles of a solid solutionof olivine and lead silicate the iron content of said olivine beingentirely in ferrous form.

9. In a battery as defined in claim 7, said zinc electrode beingpositioned on one side of said separator and including a silverelectrode positioned on the opposite side of said separator.

10. In a battery as defined in claim 8, said zinc electrode beingpositioned on one side of said separator and including a silverelectrode positioned on the opposite side of said separator.

11. A process of producing a low-gassing separator when employed with azinc electrode, and having high strength and good resistance to alkali,which comprises compacting a mixture of about 1 percent to about 65percent iron-bearing material, calculated as FeO, about 4 percent toabout 56 percent magnesium-bearing material calculated as MgO, and about0.4 percent to about 69 percent of a lead-bearing material calculated asPbO, and about percent to about 43 percent silica-bearing material, byweight, initially firing said compacted mixture at a temperature in therange of about l,l00C to about 1,400C to produce a magnesiumsilicate-iron silicate composition containing lead silicate, granulatingthe resulting compacted composition, compacting said granulatedcomposition, and sintering said last mentioned compacted composition attemperature ranging from about 1,000C to about 1,300C in a nonoxidizingatmosphere selected from the group consisting of a reducing gas and aninert gas atmosphere, under conditions to maintain the iron content ofsaid composition in the ferrous form, to produce said low- 16 gassinghigh strength separator having a porosity corresponding to a waterabsorption of about 5 percent to about 50 percent.

12. The process as defined in claim 11, said mixture containing about 14percent to about 56 percent magnesium calculated as MgO, about 1 percentto about 26 percent iron calculated as FeO, about 1 percent to about 48percent lead calculated as PbO and about 24 percent to about 32 percentsilica, by weight.

13. The process as defined in claim 11, wherein said iron-bearingmaterial is FeO, said magnesium-bearing material is MgO and saidlead-bearing material is PbO.

14. The process as defined in claim 11, wherein said initial firing alsois carried out under conditions to maintain the iron content of saidcomposition in the ferrous form.

15. The process as defined in claim 11, including incorporating anorganic binder in said mixture prior to said sintering.

16. The process as defined in claim 11, including adding about 2 percentto about 10 percent of an organic binder by weight of total inorganicsto said mixture and compacting said mixture prior to said initial firingat about l,l00C to about 1,400C, and including incorporating about 2percent to about 10 percent of an organic binder in said compactedmagnesium silicate-iron-silicate composition containing lead silicate,prior to said subsequent sintering from about 1,000C to about 1,300C.

17. The process as defined in claim 16, employing a polyethylene glycolas said binder in said mixture, and including pressing said mixture intoblocks prior to said initial firing, granulating said blocks followingsaid initial firing, incorporating said polyethylene glycol in theresulting granular mixture of magnesium silicate-iron silicatecomposition containing and pressing said last mentioned mixture intoplaques, followed by said sintering said plaques at a temperatureranging from about 1,000C to about 1,300C in said non-oxidizingatmosphere.

18. The process as defined in claim 17, wherein said initial firing alsois carried out under conditions to maintain the iron content of saidcomposition in the ferrous form.

19. The process as defined in claim 18, said starting mixture consistingessentially of about 1 percent to about 26 percent iron-bearing materialcalculated as F e0, about 14 percent to about 56 percentmagnesiumbearing material calculated as MgO, and about 1 percent toabout 48 percent of a lead-bearing material calculated as PhD, and about24 percent to about 32 percent silica, by weight.

20. A flexible battery separator as defined in claim 6, said polymericorganic binder being selected from the group consisting of afluorocarbon polymer and polyphenylene oxide.

21. A process as defined in claim 11, said nonoxidizing atmosphere beingselected form the group consisting of a balanced mixture of CO and CO acontrolled mixture of hydrogen and water vapor, nitrogen, helium andargon.

22. The process as defined in claim 11, said leadbearing material beingselected from the group consisting of lead acetate, lead dioxide, whitelead, red lead, litharge, lead hydroxide, tribasic lead silicate, leadsulfate, lead powder, lead nitrate, lead sulfide and lead carbonate.

2. A battery separator as defined in claim 1, said separator having aporosity corresponding to a water absorption ranging from about 5percent to about 50 percent.
 3. A battery separator as defined in claim2, wherein said compositIon consists essentially of about 45 to about 98mol percent magnesium silicate, about 2 to about 30 mol percent ferroussilicate, and about 0.2 to about 25 mol percent lead silicate.
 4. Abattery separator as defined in claim 3, said separator having a modulusof rupture ranging from about 5,000 to about 20, 000 psi and aresistivity ranging from about 5 to about 50 ohm-cm.
 5. A batteryseparator as defined in claim 1, wherein said composition consistsessentially of sintered particles of a solid solution of olivine andlead silicate, the iron content of said olivine being entirely inferrous form.
 6. A flexible battery separator as defined in claim 1,said sintered composition in particulate form being distributeduniformly in a polymeric organic binder insoluble in aqueous alkalinesolutions.
 7. In a battery containing a zinc electrode, a low-gassingbattery separator in the form of a porous member consisting essentiallyof sintered particles of a solid solution of about 1 to about 99 molpercent magnesium silicate, about 1 to about 90 mol percent ferroussilicate, and about 0.1 to about 50 mol percent lead silicate.
 8. In abattery as defined in claim 7, said composition of said separatorconsists essentially of sintered particles of a solid solution ofolivine and lead silicate the iron content of said olivine beingentirely in ferrous form.
 9. In a battery as defined in claim 7, saidzinc electrode being positioned on one side of said separator andincluding a silver electrode positioned on the opposite side of saidseparator.
 10. In a battery as defined in claim 8, said zinc electrodebeing positioned on one side of said separator and including a silverelectrode positioned on the opposite side of said separator.
 11. Aprocess of producing a low-gassing separator when employed with a zincelectrode, and having high strength and good resistance to alkali, whichcomprises compacting a mixture of about 1 percent to about 65 percentiron-bearing material, calculated as FeO, about 4 percent to about 56percent magnesium-bearing material calculated as MgO, and about 0.4percent to about 69 percent of a lead-bearing material calculated asPbO, and about 15 percent to about 43 percent silica-bearing material,by weight, initially firing said compacted mixture at a temperature inthe range of about 1,100*C to about 1,400*C to produce a magnesiumsilicate-iron silicate composition containing lead silicate, granulatingthe resulting compacted composition, compacting said granulatedcomposition, and sintering said last mentioned compacted composition attemperature ranging from about 1,000*C to about 1,300*C in anon-oxidizing atmosphere selected from the group consisting of areducing gas and an inert gas atmosphere, under conditions to maintainthe iron content of said composition in the ferrous form, to producesaid low-gassing high strength separator having a porosity correspondingto a water absorption of about 5 percent to about 50 percent.
 12. Theprocess as defined in claim 11, said mixture containing about 14 percentto about 56 percent magnesium calculated as MgO, about 1 percent toabout 26 percent iron calculated as FeO, about 1 percent to about 48percent lead calculated as PbO and about 24 percent to about 32 percentsilica, by weight.
 13. The process as defined in claim 11, wherein saidiron-bearing material is FeO, said magnesium-bearing material is MgO andsaid lead-bearing material is PbO.
 14. The process as defined in claim11, wherein said initial firing also is carried out under conditions tomaintain the iron content of said composition in the ferrous form. 15.The process as defined in claim 11, including incorporating an organicbinder in said mIxture prior to said sintering.
 16. The process asdefined in claim 11, including adding about 2 percent to about 10percent of an organic binder by weight of total inorganics to saidmixture and compacting said mixture prior to said initial firing atabout 1,100*C to about 1,400*C, and including incorporating about 2percent to about 10 percent of an organic binder in said compactedmagnesium silicate-iron-silicate composition containing lead silicate,prior to said subsequent sintering from about 1,000*C to about 1,300*C.17. The process as defined in claim 16, employing a polyethylene glycolas said binder in said mixture, and including pressing said mixture intoblocks prior to said initial firing, granulating said blocks followingsaid initial firing, incorporating said polyethylene glycol in theresulting granular mixture of magnesium silicate-iron silicatecomposition containing and pressing said last mentioned mixture intoplaques, followed by said sintering said plaques at a temperatureranging from about 1,000*C to about 1,300*C in said non-oxidizingatmosphere.
 18. The process as defined in claim 17, wherein said initialfiring also is carried out under conditions to maintain the iron contentof said composition in the ferrous form.
 19. The process as defined inclaim 18, said starting mixture consisting essentially of about 1percent to about 26 percent iron-bearing material calculated as FeO,about 14 percent to about 56 percent magnesium-bearing materialcalculated as MgO, and about 1 percent to about 48 percent of alead-bearing material calculated as PbO, and about 24 percent to about32 percent silica, by weight.
 20. A flexible battery separator asdefined in claim 6, said polymeric organic binder being selected fromthe group consisting of a fluorocarbon polymer and polyphenylene oxide.21. A process as defined in claim 11, said non-oxidizing atmospherebeing selected form the group consisting of a balanced mixture of CO andCO2, a controlled mixture of hydrogen and water vapor, nitrogen, heliumand argon.
 22. The process as defined in claim 11, said lead-bearingmaterial being selected from the group consisting of lead acetate, leaddioxide, white lead, red lead, litharge, lead hydroxide, tribasic leadsilicate, lead sulfate, lead powder, lead nitrate, lead sulfide and leadcarbonate.